1. Field Of The Invention
The present invention relates to endodontic instruments such as files and reamers.
2. Description Of The Prior Art
The field of endodontics involves the cleaning and enlargement of the root canals of human teeth for the purpose of removing organic material and extraneous material present in the root canal, and for enlarging the root canal so that it may be filled.
To perform such an operation, an opening is cut in the upper part of the tooth to open up the hollow portion of the tooth, and small endodontic instruments are utilized to then clean and enlarge the long narrow tapered root canals.
One very commonly used such instrument is known as a K-type file and is constructed by axially twisting a tapered triangular cross-section shaft to form three spiral cutting edges along the tapered portion of the shaft. A K-type file normally has a conical shaped tapered tip ground on the end thereof, which in accordance with standards set up by the American Dental Association and other standardizing bodies has an included angle of 75.degree. with an allowable tolerance of plus or minus 15.degree.. At the intersection of this standard 75.degree. included angle conically tapered tip with the ends of the three spiral cutting edges, relatively sharp points are created which extend laterally from the axis of the endodontic instrument.
While these standard K-type files perform entirely satisfactorily when cleaning and enlarging a straight root canal, a serious problem is often encountered when using such files to clean and enlarge a curved root canal.
As will be understood by those skilled in the art, a very large portion of the root canals encountered by a practicing endodontist are of the curved variety, and thus this problem is a significant one for the profession.
When cleaning and enlarging a root canal, the operation is performed using a set of similar endodontic files of sequentially increasing diameter. Typically an operation is begun with a file having an outside diameter at its smaller end of 0.25 millimeters. The subsequently used instruments have diameters of 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, and 0.60 millimeters. Still further instruments are available with diameters increasing in steps of 0.10 millimeter up to 1.40 millimeters for the largest size in common usage. Also, some smaller instruments are available which are sometimes used, and which have diameters of 0.10, 0.15 and 0.20 millimeters. File sizes correspond to diameter, and thus a No. 25 file has an outside diameter at its smaller end of 0.25 millimeters.
When performing an operation on a curved root canal, the operation is begun utilizing one of the smaller diameter files, such as for example a 0.25 millimeter instrument. This small diameter instrument is relatively flexible and can easily be inserted into the canal and will easily bend to fit the curved shape of the canal. With increasingly larger sizes of instruments, however, the instrument becomes significantly less flexible and becomes more and more difficult to insert through the curved portion of the canal. When utilizing the instrument to enlarge the curved portion of the canal, the force exerted along the axially outermost portion of the instrument against the root canal is very much greater against that part of the root canal to the outside of the curvature than it is against that part of the root canal on the inside of the curvature. Indeed, in many cases, the instrument will cut only on the outside of the curve and will not cut on the inside of the curvature of the root canal along those portions of the root canal near the apex.
Thus, when this problem is first encountered, the instrument being utilized will tend to cut a ledge along the outside of the curvature of the root canal at the forwardmost point in the root canal which is reached by the instrument. The succeeding instruments of greater diameter then encounter even more difficulty in conforming to the curved shaped of the canal, since it is difficult and sometimes impossible to move the tip of the next larger instrument past the ledge created by the previous instrument. Accordingly, the ledge on the outside of the curvature of the root canal is enlarged by each successively larger instrument, and ultimately the instruments very much enlarge the root canal at the point of forwardmost extent of the instruments, and in severe cases the instruments may even perforate the side of the tooth causing serious consequences for the patient.
The problem just described is often referred to as "transportation" of the instrument tip, since the tip is transported or moves laterally away from the original root canal.
This problem of transportation is also present to some extent in endodontic instruments other than the standard K-type instrument, although it is generally most severe with the K-type instrument. For example, this problem of transportation is also encountered utilizing a Hedstrom instrument which is constructed by machining one or two cutting grooves in a round tapered shaft. The Hedstrom instrument differs substantially from the K-type instrument in that the Hedstrom instrument can only cut in a single rotational direction, whereas the K-type instrument has spiral cutting edges which are rotationally bi-directional in that they may cut when rotated either clockwise or counterclockwise.
Although the prior art has appreciated the fact that transportation does occur when using endodontic instruments, and it is known that this is a serious problem which many persons have tried for a long period of time to correct, it is not believed that prior to the present invention anyone has appreciated the primary cause of lateral transportation in a K-type instrument.